1. Field of the Invention
Our invention relates generally to lighting devices having a circuit including a plurality of light emitting diodes (LEDs), and to methods of controlling and calibrating lighting devices.
2. Description of the Related Art
Lighting devices such as electric lamps, and light bulbs used therein, are well known and have been widely used for years. These lamps and bulbs are generally of the incandescent-, halogen-, and fluorescent-types. More recently, lighting devices using LEDs have entered the market. LED lighting devices have the advantages of low power requirements, high efficiency and long life. While LEDs themselves are quite efficient, the circuits that have been used to drive LED lighting devices have tended to be inefficient. For example, because readily available voltage sources tend to have much higher voltages (e.g., 12 or 24 volts DC for vehicular applications, and 120 or 240 volts for standard mains voltage from a wall socket) than the voltage required to drive an LED (2-4 volts, depending on the color), resistors and other semiconductor components have conventionally been used to regulate the voltage. In the process of regulating the voltage supplied to the LEDs, these resistors and semiconductor components convert large amounts of energy into heat, which is inefficient and potentially damaging to the LEDs and other circuit components.
Various attempts to minimize these inefficiencies have been made. For example, lighting devices are known in which a plurality of LEDs are arranged in series, so that the LEDs can be driven by a higher voltage source of power. U.S. Pat. No. 6,239,716 B1 describes, with reference to FIG. 2, a circuit having a plurality of series-connected LEDs usable in an automotive application. Each LED can be short-circuited over a switch, so that in the event of a failure of one LED, the other LEDs may continue to operate by short-circuiting the corresponding LED by means of logic circuit, or dimming of the LEDs can be performed by short-cutting the LEDs by means of circular variation. Another control circuit, which is connected to logic circuit, is connected in series with the LEDs. The logic circuit is also connected to the input of the control circuit. A constant current through the LEDs is set as current adjust by means of the logic circuit via the control circuit, regardless of how many LEDs are in operation. The voltage across all the LEDs must be adjusted upward to 60 volts, for example, in comparison with a lower on-board voltage of 12 volts or 20 volts, due to the series connection of the LEDs.
The configuration of the '716 patent requires a separate switch in parallel with each LED. Thus, if a large number of LEDs is used, a large number of switches is also required, consequently increasing the cost of the circuit. Also, as shown in FIG. 2 of the '716 patent, the control logic and control circuit are run off of approximately the same voltage as the series LED string, i.e., 60 volts, in order to close the switches. The control circuit and LEDs also share the same reference or ground. Control circuits capable of handling such relatively high voltage tend to be larger, more expensive, and to generate more heat than lower voltage (e.g., 3.3-16 volt) controllers and processors.
Accordingly, there remains a need in the art for a lighting device, which is efficient and cost effective to manufacture and operate.